How do third-party interventions affect the severity of mass killings? The authors theorize that episodes of mass killing are the consequence of two factors: (1) the threat perceptions of the perpetrators and (2) the cost of implementing genocidal policies relative to other alternatives. To reduce genocidal hostilities, interveners must address these factors. Doing so requires that interveners alter the genocidaire's expectation of a successful extermination policy, which in turn requires a demonstration of the third party's resolve. This cannot be achieved immediately upon intervention, and, given the perpetrator's strategic response to third-party involvement, the authors expect intervention to increase hostilities in the short term. With time, however, the authors contend that the characteristics of impartial interventions offer the greatest opportunity for reducing the violence in the long term. A statistical analysis of the 1955-2005 period supports the theoretical expectations. (Contains 1 figure, 4 tables, and 13 notes.)